SOLUTION

Synchronous motor is NOT a self-starting Motor.

The synchronous motor works on the principle of magnetic locking. The operating principle can be explained with the help of a 2-Pole synchronous machine with the following steps.

Let us consider a two-pole synchronous motor as shown in Figure. The three-phase supply is provided to the stator which induces two poles i.e North pole and the South pole on Stator. Since the supply in the stator is alternating in nature, therefore, its polarity changes in every half cycle, thus the poles of stator also changes after every half cycle.

The synchronous motor rotor is energized by the DC current. The field current (D.C Current) of the motor produces a steady-state magnetic field. Since the polarity of the D.C current is fixed therefore the poles of the rotor don’t vary.

Therefore, there are two magnetic fields present in the machine. Stator poles changes in every half-cycle whereas rotor poles remain the same.

Step 1. When a three-phase supply is given to the stator winding, a rotating magnetic field is produced in the stator.

Step 2.

• Due to the Rotating Magnetic field, let the stator poles i.e North poles (N_s) and South Poles (S_s) rotate with synchronous speed.
• At a particular time stator pole, N_s coincides with the rotor poles N_r and S_S coincides with S_r i.e like poles of the stator and rotor coincide with each other.
• As we know, like poles experience a repulsive force. So rotor poles experience a repulsive force F_r. Let us assume that the rotor tends to rotate in the anti-clockwise direction as shown in Fig. (i).

Step-3.

• After half cycle, the polarity of the stator pole is reversed, whereas the rotor poles cannot change their polarity as shown in Fig. (ii).
• Now unlike poles of rotor and Stator coincide with each other and rotor experiences the attractive force f_a and the rotor tends to rotate in the clockwise direction.
• In brief, we can say, with the rotation of stator poles the rotor tends to drive in the clockwise and anti-clockwise direction in every half cycle.
• Hence, to and fro motion is excited on the rotor and as a result, the rotor does not rotate. As a result, the average torque on the rotor is zero. Hence the 3-phase synchronous motor is not a self-starting motor.

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